Other factors may warrant focus
By Jordana Bieze Foster
Research presented in August at the annual meeting of the American Society of Biomechanics in Long Beach, CA, adds support to the increasingly popular idea that patellofemoral pain syndrome arises from different mechanisms—and therefore should be managed differently—in different subgroups of patients.
The findings also call into question the popular theory that patellar maltracking stems primarily from strength or activation imbalances in the vasti muscles. They suggest instead that vasti function is only one of multiple factors contributing to patellar maltracking, and it’s the other factors that may prove more useful for differentiating one PFPS subgroup from another.
Investigators from the National Institutes of Health focused on the vastus medialis (VM) muscle, which has been widely theorized to play a significant role in patellar tracking because of its function as a medial stabilizer. In 16 asymptomatic female participants, the researchers deactivated the VM (simulating that aspect of patellofemoral pain) by administering a nerve block at the femoral nerve motor branch leading to that muscle, along with a lidocaine injection. They used dynamic magnetic resonance imaging to assess patellar tracking under normal conditions and after injection.
The postinjection condition was associated with significant lateral maltracking relative to baseline, with the greatest lateral patellofemoral shift (1.8 mm) seen at a knee flexion angle of 15°. By comparison, in a separate cohort of 56 patients with patellofemoral pain, the researchers had observed 1.6 mm of lateral maltracking at the same knee angle, compared to 77 controls.
However, the postinjection volunteers did not demonstrate other characteristics seen in the patellofemoral pain cohort, including patellofemoral medial tilt and superior displacement (patella alta).
“It seems like the vastus medialis can only be part of the story,” said Frances T. Sheehan, PhD, a staff scientist in functional and applied biomechanics at the Clinical Center of the NIH in Bethesda, MD, and lead author of the study.
Perhaps not coincidentally, the same variables that differentiated the postinjection individuals from the true patellofemoral pain patients also differ between subgroups within that PFPS cohort based on lateral or nonlateral maltracking. At 15° of knee flexion, the 35 lateral maltrackers had a lateral PF tilt of 3.9°, while the 21 nonmedial maltrackers had a medial PF tilt of 5°. Lateral maltrackers demonstrated a 4.1-mm superior displacement, while nonmedial maltrackers had no evidence of alta.
The findings suggest that at least two pathways are responsible for patellofemoral pain, Sheehan proposed. A combination of VM impairment and ligament laxity could lead to lateral maltracking, while VM impairment plus increased contact force against the lateral femoral sulcus could lead to nonlateral maltracking.
In another ASB presentation, researchers from Stanford University also found that vastus function correlated with one subgroup of PFPS patients but not another. In 54 individuals with PFPS and 15 controls, the investigators used EMG to assess the vastus lateralis-vastus medialis (VL:VM) activation ratio and VM activation delay as participants walked at self-selected speeds.
They found significant relationships between VL:VM ratio and both VM activation delay and patellar tilt, but only in the subset of patients classified as maltrackers based on MRI. In the subset of patients with normal tracking, VL:VM activation ratio was unrelated to VM activation delay or patellar tilt.
The findings are consistent with those of a previous study from the same group, published in the American Journal of Sports Medicine in March, which found a significant relationship between VM activation delay and patellar tilt in maltrackers but not in normal trackers.
One explanation for the discrepancy might be that increased joint conformity and highly functional patellofemoral ligaments in normal-tracking individuals minimize the effects of vasti impairment on patellar tracking, the authors said.